Modified cascade model of resonant Raman scattering: a case study of UV Raman scattering in Zn1−xMnxO thin films



The cascade model of inelastic resonant Raman scattering considers real electronic states in the conduction band (CB) as intermediaries to explain multiple longitudinal optical (LO) Stokes-shifted lines in the emission spectra. In this study, we report modification in the cascade model under conditions where the scattered photons after multiple transitions have energy lower than the bandgap (Eg) and give rise to higher order n-LO lines. The higher order n-LO lines involve electron transition between the trap levels, which are created by impurities or defects in the forbidden energy gap, and are analogous to the real electronic states in CB, depending on the density of defects or impurities in the lattice. The presence of traps in the forbidden gap (1) acts as virtual intermediate states giving rise to higher order n-LO modes and (2) tends to decrease the radiative recombination probability leading to quenching of the luminescence emission and line width (full-width at half-maximum) broadening. Ultraviolet Raman scattering in Mn-doped ZnO (Zn1−xMnxO) thin films were investigated and the experimental observations analyzed in the domain of the modified cascade model. Copyright © 2011 John Wiley & Sons, Ltd.